The present invention relates to a non-volatile memory device, and more particularly, the present invention relates to programming and reading a non-volatile memory device.
In non-volatile memory devices, data is retained even when power is removed. Examples of non-volatile memory devices include mask read-only memory (ROM) (MROM), electrically erasable programmable ROM (EEPROM), and the like.
A flash memory device, in particular, may have a different configuration and operation than a conventional EEPROM. The concept of a flash memory device was introduced as a flash EEPROM in 1984 by Toshiba. A flash memory device may perform an electric erase operation by block, sector or chip units. Further, the flash memory device may be configured to perform a program operation by one bit units. The flash memory device may have architecture similar to a disk-type auxiliary storage device having a storage area divided and formatted in sector units.
A flash memory device may be classified as a NOR-type flash memory device or a NAND-type flash memory device. In the NOR-type flash memory device, memory cells are connected in parallel between a bit line and a ground line. In the NAND-type flash memory device, memory cells are connected in series between a bit line and a ground line. The NOR-type flash memory device has various advantages, including a simple peripheral circuit and a short read access time. However, since the NOR-type flash memory device requires contact with a bit line per cell, it typically needs a larger cell area than the NAND-type flash memory device. In comparison, since memory cells of the NAND-type flash memory device are connected in series to a bit line, a read speed of the NAND-type flash memory device may be relatively slow as compared to the NOR-type flash memory device.
Threshold voltages of programmed memory cells in a flash memory device may change or shift due for various reasons, such as incremental step pulse programming (ISPP) increments, floating gate coupling, charge loss and the like. This may deteriorate reliability during read operations.